Positive and negative photoresists are used in the semiconductor fabrication art to pattern the material layers that are formed on silicon substrates to construct integrated circuits. In the conventional practice, a material layer for the integrated circuits is formed over the substrate, or is formed over other material layers that have been previously formed over the substrate. Next, a photoresist layer, which may be a positive type, a negative type, or an image-reversal type, is formed over the material layer by spin coating, followed by soft-baking at around 90.degree. C. to 110.degree. C. Thereafter the photoresist layer is exposed to ultraviolet light (actinic radiation) through a pattern mask, which replicates a pattern on the photoresist which may be removed by a developer solution. For positive resists, the replicated pattern is substantially identical to that of the mask, and is a positive image. For negative photoresists, the replicated pattern is the negative, or reverse of the mask pattern. The photoresist is then exposed to the developer to remove the replicated pattern, thereby leaving portions of the underlying material layer exposed. Developers for positive photoresist usually comprise a moderately basic solution (aqueous alkaline solution) at a pH of between 10 and 12. Developers for negative photoresist usually comprise an organic solvent. Depending upon its composition, the photoresist layer is usually then baked at a temperature between 100.degree. C. and 150.degree. C. (i.e., post-baked) to improve its stability. After the photoresist layer has been developed and optionally post-baked, the substrate is dipped in an etchant solution which attacks the exposed portions of the underlying material layer but not the photoresist. After the material layer has been sufficiently etched, the substrate is rinsed, and the photoresist layer is removed by exposure to a stripper solution, which typically comprises a relatively toxic organic solvent and which must be disposed of in a highly specialized and expensive manner. A typical stripper comprises n-butyl acetate, which is toxic and flammable.
The present invention seeks to modify the use of the photoresist layer in the above-described conventional patterning process so that the photoresist may be stripped by using chemical solutions which are non-toxic or which can be neutralized to be non-toxic, thereby making the above-described patterning process more environmentally friendly.